This invention relates generally to apparatus for printing information on recording material such as thermal or electrosensitive paper and, more particularly, to a mechanism which utilizes a single stepper motor to control both the motion of the recording material past a print head and also the retraction of the print head from the recording material surface as the material is moved.
Printers for generating hard copy readout on recording material such as thermal or electrosensitive paper are used extensively for electronic devices such as data terminals, calculators, cash registers and the like. Such information takes the form of alpha-numeric characters as well as symbolic and graphic printouts.
Typically, in such printing apparatus, a print head is moved across a web of recording material to print a line of information onto the web of material. The web is then advanced so that the next line of information can be printed. Both the print head and the recording material are moved rapidly in the printing apparatus to accommodate the rapid printing speeds required by modern-day users. Often the print head includes needle-like elements organized into a matrix to print the required information or other projecting elements which may catch or snag the recording material as the material is moved past the print head. Contact between the print head and the recording material causes unsightly marks on the material and wears away the print head over time. To prevent such problems, the print head is often lifted from the recording material as the material is advanced.
Various mechanisms for lifting the print head and moving the recording material have been provided in the prior art. For example, a thermal printer is disclosed in U.S. Pat. No. 3,845,850 which was issued on Nov. 5, 1974 to Herr et al. The printer includes a print head which is moved laterally across a sheet of thermally sensitive paper backed by a roller or platen. The print head is moved by a stepper motor. Over the majority of its travel, the print head is held in contact with the thermally sensitive paper. Toward one end of its travel, a projection from a print head carriage engages a ramp which lifts the head from the thermally sensitive paper as the carriage is moved.
If the print head carriage is driven beyond the print head lifting point, a ratchet mechanism rotates the platen to advance the thermally sensitive paper by one line increment. In the mechanism disclosed in this patent, the recording paper can be advanced and the print head raised only when the print head is at one side of the recording paper. Also, the recording paper can be moved in only one direction through the printer.
A second recording material advancing mechanism for a thermal printer is disclosed in U.S. Pat. No. 3,787,886 issued on Jan. 22, 1974 to McCrady. In this printer, a single solenoid activates two rocking mechanisms. The first rocking mechanism includes a one way clutch arrangement to pinch the recording material when rocked in one direction by operation of the solenoid and to thereby draw the material in that direction. The material is released when the first rocking mechanism is rocked in the other direction to return the first rocking mechanism to its initial position without affecting the position of the recording material. The second rocking mechanism comprises a print head which is retracted from the recording material upon activation of the solenoid.
The mechanism disclosed in this patent simultaneously advances the recording material while releasing the print head from engagement with the material. Here again, the material can only be moved in one direction through the printer and the simultaneous retraction of the print head from the recording material may not prevent marking or snagging of the material by the print head in all cases.
A third recording material advance mechanism is disclosed in U.S. Pat. No. 3,955,663 issued on May 11, 1976 to Ecker. In this mechanism, a single solenoid initially advances the recording material by one line increment and then forces the material against a printing head. Here again, the mechanism can only provide for movement of the recording material in one direction through the printing machine and is mechanically complex.
Thus, the need exists for an improved mechanism for coordinating the motion of a print head as material to be printed upon is moved within a printing device.